where \({N_{\text{C}}}\)  is the number of the active transmission antennas, \({\rho _{\text{C}}}\)  is the power consumed by circuits associated with an active transmission antenna, \(\rho \)  is the transmit power of a transmission antenna, \(\alpha \)  is the slope of the load dependent power consumption. Comparing to transmitters, the power consumed by the receiver is very small and negligible [1]}, [2]}. Therefore, only the power consumed by transmitters is considered for OAM-SM and MIMO communication systems. In general, the energy efficiency of communication systems is expressed by
\(\eta = \frac{{W \cdot C\left( {{N_{\text{C}}}\rho } \right)}}{{{\rho ^{{\text{total}}}}}},\qquad \mathrm {(42)}\) 
